1. Field of the Invention
The present invention relates to a wavelength division multiplexing optical transmission system and a wavelength division multiplexing optical transmission method. More specifically, the present invention relates to a wavelength division multiplexing optical transmission system and a wavelength division multiplexing optical transmission method, both of which make it possible to make a residual dispersion value in a received optical signal an optimal value.
2. Description of the Related Art
An “optical transmission” method of transmitting, and receiving, information as a light intensity modulation signal has been used for the purpose of transmitting a large amount of data in a long distance in recent years. In the case of the optical transmission method, an optical transmitter converts an electrical signal to an optical signal (E/O conversion), and transmits the optical signal to an optical receiver. The optical receiver converts the received optical signal to an electrical signal (O/E conversion), and thereby obtains the original information. A communications system using an optical fiber as a transmission path has been generally known as a communications system employing the optical transmission method. In the case of the optical fiber communications system, a method of transmitting an optical signal which is obtained by multiplexing the signals on a single optical fiber has been used in order to increase the amount of information to be transmitted in each optical fiber.
A method of multiplexing signals while the signals are still in the form of electrical signals and a method of multiplexing signals after the signals are converted to optical signals are applicable to the optical transmission. As the former method, Time Division Multiplexing (TDM) and Frequency Division Multiplexing (FDM) are known. As the latter method, Space Division Multiplexing (SDM) and Wavelength Division Multiplexing (WDM) are known. Out of these methods, WDM is a method of causing a plurality of optical signals each with a different wavelength to be transmitted with a single optical fiber. Since WDM can use the existing optical fiber networks, WDM is economically advantageous, and is put into practical use widely throughout the world.
In the case of the WDM optical transmission system, signals are deteriorated stemming from chromatic dispersion in the transmission path (optical fiber). In other words, since a luminance element for the E/O conversion in an optical transmitter is a light source-whose spectrum spreads, each of optical signals to be sent out from the optical transmitter actually has some degree of a bandwidth. Optical signals with the respective different wavelengths (i.e., different frequencies) are different from one another in speed at which each of the optical signals travels through the optical fiber. For this reason, as the transmission distance becomes longer, phases are shifted depending on the frequency components even in a single optical signal. This constitutes a cause of bit errors.
For this reason, in the WDM optical transmission system, compensation is needed to return, to zero, chromatic dispersion suffered in the optical fiber transmission path for the purpose of inhibiting the signal from being deteriorated. United States Patent Application Publication No. US 2003/0095766A1 has disclosed an optical transmission system in which a dispersion compensation fiber is connected to a transmission path optical fiber in each span, and in which sufficient dispersion compensation is performed over the entire optical transmission path.
Descriptions will be provided for an example of a dispersion map of a conventional WDM optical transmission system with reference to FIG. 1A and FIG. 1B. As shown in FIG. 1A, in this WDM optical transmission system, optical signals from an optical transmitter are transmitted to an optical receiver through some relay nodes. The wavelengths are dispersed in the transmission path optical fiber in each of spans respectively between the optical transmitter and the nearest relay node, between each two neighboring relay nodes, and between the optical receiver and the nearest relay node.
For this reason, the dispersion is compensated by 100% in each of the spans by use of a Dispersion Compensation Fiber (DCF). The DCF has a dispersion value which is equal, in absolute value, to the dispersion of the span of the transmission path optical fiber, and whose sign is opposite to the dispersion of the span of the transmission path optical fiber. How the chromatic dispersion is compensated by use of the DCFs is shown in the dispersion map of FIG. 1B. The horizontal axis represents the transmission distance, and corresponds to each of the transmission spans shown in FIG. 1A. In the case of this compensation method, the WDM signal needs to be compensated collectively. For this reason, the dispersion slope of the transmission path optical fiber should be considered to compensate the chromatic dispersion by 100% in each of the transmission spans. With regard to transmission path fibers each with a higher dispersion slope, including some of Dispersion Shifted Fibers (DSFs), it is difficult for the dispersion to be compensated by 100% over the entire wavelengths of the WDM signal by use of the DCFs.
In addition, an LN (LiNbO3) optical modulator has been generally used as an optical modulator in an optical transmitter in the case where the long-distance transmission is performed. In general, the LN modulator is operated in a way that each of the optical signals has a negative chirp coefficient. The reason for this is that the dispersion is intended to be easily compensated for the purpose of inhibiting change in waveforms after performing the optical fiber transmission. The change in waveforms stems from combination of the Self Phase Modulation (SPM) and the chromatic dispersion. For this reason, with regard to the signals to be received by the optical receiver through the optical fiber, an optimum residual dispersion value does not take on zero, but a specific value. Incidentally, the optimum residual dispersion value is defined as the dispersion value which makes the bit error rate the smallest when test patterns (bit patterns) are sent out from the optical transmitter.
This point will be described in detail. Theoretically, when the residual dispersion value is returned to zero by means of completely compensating the dispersion, the signal takes the same waveform as the signal took when the signal was transmitted. However, when the dispersion is provided thereto, a phenomenon additionally occurs in which the bit error rate is further improved. This phenomenon occurs because of phase modulation accompanied by intensity modulation called “chirp”. In a case where the chirp is present, the optical pulse is compressed due to dispersion accumulation. This improves the receiver sensitivity.
The chirp which causes the aforementioned phenomenon is a kind of phase modulation. The phase modulation includes the chirp which is provided due to characteristics of the optical transmitter at a time of transmission. In addition to the chirp, the phase modulation also includes nonlinear phase modulation which is provided due to an optical nonlinear effect in the transmission path optical fiber. The optical nonlinear effect is a phenomenon in which the refractive index of an optical fiber changes depending on the instantaneous optical power. Due to this phenomenon, the optical signal transmitted through the optical fiber suffers the phase modulation. The nonlinear phase modulation effects in the WDM system include Self Phase Modulation (SPM) stemming from its own channel and Cross Phase Modulation (XPM) stemming from other propagating channels in parallel. These nonlinear phase modulation effects occur at a moment when an optical signal is made incident onto the optical fiber transmission path from the optical amplifier. For this reason, the cumulative amount of the nonlinear phase modulation effects varies depending on the launched optical power into the optical fiber and the number of relays (i.e., repeat) of the optical amplifier, etc.
Even if the dispersion compensation fiber is designed to return the residual dispersion value to zero (achieve the 100% compensation), the amount of the chromatic dispersion varies from one wavelength to another, as shown in FIG. 2. In other words, the transmission path has a dispersion slope. For this reason, in a case where a dispersion compensation fiber which does not match the compensation dispersion value and the transmission path dispersion value with each other is used, it is difficult to cause all of the residual dispersion values of all the wavelength components to be equal to intended values.
By use of FIG. 3A and FIG. 3B, descriptions will be provided for causes of a compensation error which occurs in a case where a residual dispersion value in the optical receiver is not caused to return to zero in the conventional WDM optical transmission system. In the transmission system shown in FIG. 3A, it is supposed that the residual dispersion value at the optical receiver is the intended particular value other than zero as shown in a dispersion map in FIG. 3B. However, it is difficult to cause all of the residual dispersion values of all the wavelength components to be equal to the intended value, even if the optical receiver is designed not to compensate the chromatic dispersion in the last span by 100%. In other words, in a case where a dispersion compensation fiber which does not match the compensation dispersion value and the transmission path dispersion value with each other is used, the residual dispersion value of a particular wavelength component may be equal to the intended value, but the residual dispersion values of the other wavelength components are not equal to the intended values. As a consequence, the residual dispersion in the WDM bandwidth causes a compensation error in response to the dispersion slope in the transmission path fiber.
Particularly, in a case where the optical fiber as the transmission path is a DSF, this decreases the rate of compensation of the dispersion slope in the DCF. Accordingly, this brings about a problem of a large compensation error occurring in the residual dispersion. As described above, in the conventional WDM optical transmission system, it has been difficult to simultaneously achieve the causing of the targeted residual dispersion value (not “zero”) of the received optical signal and the decreasing of the dispersion compensation error.
Incidentally, another related art have been disclosed in U.S. Pat. No. 6,324,317B1 or US Patent Application Publication No. US2002/0101633A1. In the case of these techniques, however, a negative preset dispersion is provided, and a final amount of the accumulated chromatic dispersion is in the negative region. The precondition for these techniques is different from the precondition for the present invention. Thus, no method of solving the aforementioned problems has been disclosed in these related arts.